The invention disclosed and claimed herein pertains generally to bootstrap loader programs and associated equipments for use in a computer system to enable changing or alteration of discrete program instructions. The invention further pertains to bootstrap loader equipment which is nonvolatile, i.e., which retains a bootstrap program when the computer system power is turned off that otherwise erases all software stored in the system's main memory.
A data processing or computer system performs specified functions by storing a number of programs in a system main memory, and appropriately directing a system central processing unit (CPU). Consequently, a procedure is included in the initialization of the computer system for entering a bootstrap loader into the system main memory. Since a bootstrap loader is a program which directs the CPU to load other programs into a main memory from floppy discs, magnetic tape, or like sources, all system software except the bootstrap program is automatically loaded by routine CPU operation.
In a number of conventional systems, a bootstrap loader program is entered into a main memory by means of a keyboard or a paper tape reader/punch. For example, to enter a bootstrap program into the main memory of a minicomputer such as the Varian V73, a high speed paper tape reader is coupled to the I/O bus and a bootstrap loader control in the CPU is energized. The hard-wired load control is capable of generating enough control signals, the bootstrap loader program, to cause successive instructions on the tape to be read off the tape and into the main memory.
A readily apparent disadvantage of the above conventional procedure is a relatively expensive paper tape peripheral device whether or not such device has any other use in the computer system operation. Also, since a paper tape reader is an electro-mechanical device having many moving parts and a slow rate of operation time is lost from computer operation. A further disadvantage in a paper tape bootstrap loader system is its vulnerability to a main memory power loss that erases all software, including any previously entered bootstrap loader program, is from the main memory. Additional time is then lost in reinitializing the main memory by means of a paper tape reader.
In order to overciome the above volatility problem, a prior art device includes a set of fixed or "hard-blown" read only memories (ROMs) configured to have a particular bootstrap program that is invulnerable to a main memory or system power loss. However, changing even a single instruction in the set program takes hours of time to obtain and substitute new ROM's.
Through their invention, the Applicants provide a programmable or alterable bootstrap loader peripheral device. This device can be employed with any conventional data processing or computer system to rapidly enter a bootstrap program into a system main memory, without needing a paper tape or a fixed ROM. The invention allows particular instructions included in the bootstrap program to be quickly and simply altered before or after the program is entered into the main memory. At the same time, the bootstrap loader device of Applicants' invention retains a nonvolatile bootstrap program viable after power is shut off to enable a later, rapid re-initialization of a computer system.